问一道模拟提中的题目。

holmzUID: 178937

10. Two positive charges of q and 2q coulombs are located on the x-axis at x = 0.5a and 1.5a, respectively, as shown above. There is an infinite, grounded conducting plane at x = 0. What is the magnitude of the net force on the charge q?
(A)         
(B)         
(C)         
(D)         
(E)         
答案是：（1/4pi%）(3q*q)/(a*a),可是我觉得是：（1/4pi%）(7q*q)/(2a*a),